1. Field of the Invention
The present invention relates to open loop control for semiconductor lasers, and particularly, mirror, gain and phase current control for Sampled Grating Distributed Bragg Reflector (SGDBR) semiconductor lasers.
2. Description of the Related Art
Diode lasers are being used in such applications as optical communications, sensors and computer systems. In such applications, it is very useful to employ lasers that can be easily adjusted to output frequencies across a wide wavelength range. A diode laser which can be operated at selectably variable frequencies covering a wide wavelength range, i.e. a widely tunable laser, is an invaluable tool. The number of separate channels that can utilize a given wavelength range is exceedingly limited without such a laser. Accordingly, the number of individual communications paths that can exist simultaneously in a system employing such range-limited lasers is similarly very limited. Thus, while diode lasers have provided solutions to many problems in communications, sensors and computer system designs, they have not fulfilled their potential based on the available bandwidth afforded by light-based systems. It is important that the number of channels be increased in order for optical systems to be realized for many future applications.
For a variety of applications, it is necessary to have tunable single-frequency diode lasers which can select any of a wide range of wavelengths. Such applications include sources and local oscillators in coherent lightwave communications systems, sources for other multi-channel lightwave communication systems, and sources for use in frequency modulated sensor systems. Continuous tunability is usually needed over some range of wavelengths. Continuous tuning is important for wavelength locking or stabilization with respect to some other reference, and it is desirable in certain frequency shift keying modulation schemes.
In addition, widely tunable semiconductor lasers, such as the sampled-grating distributed-Bragg-reflector (SGDBR) laser, the grating-coupled sampled-reflector (GCSR) laser, and vertical-cavity lasers with micro-mechanical moveable mirrors (VCSEL-MEMs) generally must compromise their output power in order to achieve a large tuning range. The basic function and structure of SGDBR lasers is detailed in U.S. Pat. No. 4,896,325, issued Jan. 23, 1990, to Larry A. Coldren, and entitled xe2x80x9cMULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENT MIRRORSxe2x80x9d, which patent is incorporated by reference herein. Designs that can provide over 40 nm of tuning range have not been able to provide much more than a couple of milliwatts of power out at the extrema of their tuning spectrum. However, current and future optical fiber communication systems as well as spectroscopic applications require output powers in excess of 10 mW over the full tuning band. Current International Telecommunication Union (ITU) bands are about 40 nm wide near 1.55 xcexcm, and it is desired to have a single component that can cover at least this optical bandwidth. Systems that are to operate at higher bit rates will require more than 20 mW over the full ITU bands. Such powers are available from distributed feedback (DFB) lasers, but these can only be tuned by a couple of nanometers by adjusting their temperature. Thus, it is very desirable to have a source with both wide tuning range ( greater than 40 nm) and high power ( greater than 20 mW) without a significant increase in fabrication complexity over existing widely tunable designs. Furthermore, in addition to control of the output wavelength, control of the optical power output for a tunable laser is an equally important endeavor as optical power determines the potential range for the laser.
Fundamentally, maximizing the output power, while stabilizing the output power and wavelength and maximizing the side mode suppression ratio are very desirable objectives in the control of SGDBR lasers. Thus, there is a need in the art for devices and methods which maximize the power output, and stabilize power and wavelength output. The present invention meets these objectives through a novel use of open loop control.
The present invention involves the open loop control of the frequency and power output of a Sampled Grating Distributed Bragg Reflector (SGDBR) semiconductor laser. The open loop control of such SGDBR devices provides stable SGDBR laser optical power and wavelength output.
The open loop control of the present invention uses a table of voltages and current settings to control the optical output power and the output wavelength or frequency. Once the optical power and output wavelength are selected, the open loop controller of the present invention selects a set of operating currents and voltages from the table corresponding to the selected output power and output wavelength. Further, the open loop controller regulates the temperature of the SGDBR laser to a fixed, pre-selected value.
To generate the operating currents, each SGDBR laser is calibrated using a calibration routine, and each controller is programmed with the values for the corresponding laser, which then controls the laser over the lifetime of the SGDBR laser.
By properly choosing the operating currents, the current sources that deliver the currents to the SGDBR laser, and properly regulating the temperature of the SGDBR laser, the open loop controller of the present invention provides greater stability of the optical output wavelength and power over the operating lifetime, as well as providing greater stability over a wider range of ambient environmental conditions.